Generally, for convenience and because standard semiconductor tools can be used in the fabrication process, radiation masks, e.g. projection electron beam lithography masks, X-ray masks, etc., are formed on a semiconductor wafer, such as a silicon wafer. The silicon wafer operates like a frame and support for the mask. A thin layer that ultimately becomes the membrane is deposited on the upper surface of the wafer. A layer of beam scattering material for projection electron beam lithography or radiation absorbing material for X-rays, such as some safe heavy metal or alloy, is deposited on the upper surface of the membrane layer. For the case of X-ray masks, the radiation absorbing layer is patterned by applying a hard mask material and resist. The resist is patterned (or exposed) with an electron beam (E-beam) device and a hard mask is formed by etching the hard mask layer through the patterned resist layer. The hard mask is then used as an etch mask to pattern the radiation absorbing layer. Patterning of projection electron beam scattering layers is similar, except that the intermediate hardmask layer is usually eliminated because of the reduced thickness of the metal layer. At some point in the process the wafer is etched from the membrane layer in a pattern to form one or more thin membranes. The mask thus allows radiation to pass through the thin membrane and portions of the radiation absorbing or scattering layer that have been etched away. The entire procedure is known as a process flow and two different process flows are commonly used.
In the first process flow, commonly referred to as a wafer flow, all processing is done on the wafer with one of the final steps being the back etching of the silicon wafer to form the membrane. The wafer flow was primarily created to solve formatting issues. It allows refractory radiation mask processing to be conducted in semiconductor tools that are not dramatically different from the standard wafer processing tools supplied by the industry. The refractory radiation mask specific processing steps (membrane formation and wafer mounting to a support ring, if employed) are at the end of the flow. This minimizes the modifications necessary to both the tools and the wafer processes. However, the creation of the membrane and the mounting of the wafer can create significant pattern displacement errors in the mask.
The second process flow is commonly referred to as a membrane flow. In the membrane flow the membrane is formed early in the process (generally after hard mask deposition) and the remaining processing is carried out on the membrane. The membrane flow process was derived to remove the errors in the wafer flow process by conducting the mask specific processing steps before the scattering or absorbing layer is patterned. While this greatly reduces the errors associated with membrane formation and wafer mounting, it also increases the modifications to both the equipment (the tools must accept a refractory radiation mask format rather than a wafer) and the processes (the patterning defining process steps are carried out on a membrane rather than a wafer).
Many properties are considered when selecting a membrane material, but the important ones from the standpoint of silicon etching are the chemical resistance of the membrane material to the liquid etchant (e.g. hot KOH) and overetch times. For thinner membranes (1500 .ANG. and less, as in the case of projection electron beam lithography masks), these issues become far more significant. The use of an electrochemical etch stop layer generally is useful only in the formation of thicker membranes (20-25 .mu.m) as this method does not have the control or repeatability required for the formation of thinner membranes.
Accordingly, it would be advantageous to have an endpoint system in-situ that could determine when the silicon etch has been completed, thereby minimizing overetch times.
It is a purpose of the present invention to provide new and improved methods of in-situ endpoint detection during membrane formation.
It is another purpose of the present invention to provide new and improved methods of in-situ endpoint detection during the fabrication of radiation masks.
It is still another purpose of the present invention to provide new and improved methods of in-situ endpoint detection during membrane formation, and especially thinner membrane formation, e.g. less than 10,000 .ANG..
It is a further purpose of the present invention to provide new and improved methods of in-situ endpoint detection during membrane formation which are fully compatible with various silicon etch chemistries as well as any choice of membrane material.
It is a still further purpose of the present invention to provide new and improved methods of in-situ endpoint detection during membrane formation which are extendible to various sensor applications.